1. Field of the Disclosure
The present invention relates generally to control circuits that regulate the delivery of energy in a switched mode power converter and, more specifically, the present invention relates to control circuits that reduce the energy consumption of switched mode power converters under light or no load conditions by use of an unregulated dormant mode of operation.
2. Background
Power converter control circuits may be used for a multitude of purposes and applications. There is a demand for control circuit functionality that can reduce the energy consumption of the power converter. In particular, there is specific demand for control circuits that reduce the energy consumption of power converters under conditions of light or no load. This demand comes from the fact that in some applications of power converters there is little or no requirement for energy delivery at all for long periods of time. One example of such an application is in AC-DC chargers for cellular telephones. The AC-DC charger is often left connected to the AC mains outlet in the home or office, even when the cellular phone itself is completely disconnected from the output cable of the AC-DC charger. Such a condition is often referred to as a no-load condition. Furthermore in applications such as cellular telephones and digital still cameras and the like, the unit being powered by the output of the AC-DC charger shuts down once the battery internal to the unit is fully charged. Under these conditions the energy requirement of the unit falls dramatically and is therefore a very light load condition for the AC-DC charger. This condition is often referred to as a standby or sleep mode and again can exist for long periods. There is therefore also demand for the AC-DC charger to operate with high efficiency or in other words with the lowest possible energy consumption under these very light load standby or sleep mode conditions.
Existing control circuits for switched mode power converters typically reduce the energy consumption of the power converter by reducing the switching frequency of a power switch coupled to the control circuit, to reduce a type of energy loss called switching losses. During this time of reduced switching frequency, the control circuits remain active by maintaining a power converter output voltage such that the unit to be powered (the cellular telephone handset or digital still camera for example) can receive energy as soon as it is connected to the AC-DC charger output or as soon as it comes out of sleep/standby mode and demands more energy.